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class="icon minus" data-v-2976c796><path d="M19,2H5C3.3,2,2,3.3,2,5v14c0,1.7,1.3,3,3,3h14c1.7,0,3-1.3,3-3V5C22,3.3,20.7,2,19,2zM20,19c0,0.6-0.4,1-1,1H5c-0.6,0-1-0.4-1-1V5c0-0.6,0.4-1,1-1h14c0.6,0,1,0.4,1,1V19z"></path><path d="M16,11H8c-0.6,0-1,0.4-1,1s0.4,1,1,1h8c0.6,0,1-0.4,1-1S16.6,11,16,11z"></path></svg><svg version="1.1" xmlns="http://www.w3.org/2000/svg" viewbox="0 0 24 24" class="icon plus" data-v-2976c796><path d="M19,2H5C3.3,2,2,3.3,2,5v14c0,1.7,1.3,3,3,3h14c1.7,0,3-1.3,3-3V5C22,3.3,20.7,2,19,2z M20,19c0,0.6-0.4,1-1,1H5c-0.6,0-1-0.4-1-1V5c0-0.6,0.4-1,1-1h14c0.6,0,1,0.4,1,1V19z"></path><path d="M16,11h-3V8c0-0.6-0.4-1-1-1s-1,0.4-1,1v3H8c-0.6,0-1,0.4-1,1s0.4,1,1,1h3v3c0,0.6,0.4,1,1,1s1-0.4,1-1v-3h3c0.6,0,1-0.4,1-1S16.6,11,16,11z"></path></svg></div></div><div class="items" data-v-2976c796><!--[--><!--[--><a class="VPLink link link" href="/articles/rust/01.%E5%9F%BA%E7%A1%80%E5%85%A5%E9%97%A8.html" style="padding-left:0px;" data-v-2cfe069a data-v-3c355974><!--[--><span class="link-text" data-v-2cfe069a>01.基础入门</span><!--]--><!----></a><!----><!--]--><!--[--><a class="VPLink link link" href="/articles/rust/02.%E5%9F%BA%E7%A1%80%E8%AF%AD%E6%B3%95.html" style="padding-left:0px;" data-v-2cfe069a data-v-3c355974><!--[--><span class="link-text" data-v-2cfe069a>02.基础语法</span><!--]--><!----></a><!----><!--]--><!--[--><a class="VPLink link link active" href="/articles/rust/03.%E6%8C%87%E9%92%88%E4%B8%8E%E5%86%85%E5%AD%98%E5%88%86%E9%85%8D.html" style="padding-left:0px;" data-v-2cfe069a data-v-3c355974><!--[--><span class="link-text" data-v-2cfe069a>03.指针与内存分配</span><!--]--><!----></a><!----><!--]--><!--[--><a class="VPLink link link" href="/articles/rust/04.%E7%A8%8B%E5%BA%8F%E7%BB%93%E6%9E%84%E5%92%8C%E5%BC%82%E5%B8%B8%E5%A4%84%E7%90%86.html" style="padding-left:0px;" data-v-2cfe069a data-v-3c355974><!--[--><span class="link-text" data-v-2cfe069a>04.程序结构和异常处理</span><!--]--><!----></a><!----><!--]--><!--[--><a class="VPLink link link" href="/articles/rust/05.%E6%95%B0%E6%8D%AE%E7%BB%93%E6%9E%84%E4%B8%8E%E6%B3%9B%E5%9E%8B.html" style="padding-left:0px;" data-v-2cfe069a data-v-3c355974><!--[--><span class="link-text" data-v-2cfe069a>05.数据结构与泛型</span><!--]--><!----></a><!----><!--]--><!--[--><a class="VPLink link link" href="/articles/rust/06.trait%E4%B8%8E%E7%94%9F%E5%91%BD%E5%91%A8%E6%9C%9F.html" style="padding-left:0px;" data-v-2cfe069a data-v-3c355974><!--[--><span class="link-text" data-v-2cfe069a>06.trait与生命周期</span><!--]--><!----></a><!----><!--]--><!--[--><a class="VPLink link link" href="/articles/rust/07.%E7%BC%96%E5%86%99%E6%B5%8B%E8%AF%95.html" style="padding-left:0px;" data-v-2cfe069a data-v-3c355974><!--[--><span class="link-text" data-v-2cfe069a>07.编写测试</span><!--]--><!----></a><!----><!--]--><!--[--><a class="VPLink link link" href="/articles/rust/08.%E6%9E%84%E5%BB%BA%E5%91%BD%E4%BB%A4%E8%A1%8C%E7%A8%8B%E5%BA%8F.html" style="padding-left:0px;" data-v-2cfe069a data-v-3c355974><!--[--><span class="link-text" data-v-2cfe069a>08.构建命令行程序</span><!--]--><!----></a><!----><!--]--><!--[--><a class="VPLink link link" href="/articles/rust/09.%E9%97%AD%E5%8C%85%E4%B8%8E%E8%BF%AD%E4%BB%A3%E5%99%A8.html" style="padding-left:0px;" data-v-2cfe069a data-v-3c355974><!--[--><span class="link-text" data-v-2cfe069a>09.闭包与迭代器</span><!--]--><!----></a><!----><!--]--><!--[--><a class="VPLink link link" href="/articles/rust/10.%E5%8F%91%E5%B8%83%E9%A1%B9%E7%9B%AE.html" style="padding-left:0px;" data-v-2cfe069a data-v-3c355974><!--[--><span class="link-text" data-v-2cfe069a>10.发布项目</span><!--]--><!----></a><!----><!--]--><!--[--><a class="VPLink link link" href="/articles/rust/11.%E6%99%BA%E8%83%BD%E6%8C%87%E9%92%88.html" style="padding-left:0px;" data-v-2cfe069a data-v-3c355974><!--[--><span class="link-text" data-v-2cfe069a>11.智能指针</span><!--]--><!----></a><!----><!--]--><!--[--><a class="VPLink link link" href="/articles/rust/12.%E5%B9%B6%E5%8F%91.html" style="padding-left:0px;" data-v-2cfe069a data-v-3c355974><!--[--><span class="link-text" data-v-2cfe069a>12.并发</span><!--]--><!----></a><!----><!--]--><!--[--><a class="VPLink link link" href="/articles/rust/13.%E9%9D%A2%E5%90%91%E5%AF%B9%E8%B1%A1.html" style="padding-left:0px;" data-v-2cfe069a data-v-3c355974><!--[--><span class="link-text" data-v-2cfe069a>13.面向对象</span><!--]--><!----></a><!----><!--]--><!--[--><a class="VPLink link link" href="/articles/rust/14.%E4%B8%8D%E5%AE%89%E5%85%A8Rust.html" style="padding-left:0px;" data-v-2cfe069a data-v-3c355974><!--[--><span class="link-text" data-v-2cfe069a>14.不安全Rust</span><!--]--><!----></a><!----><!--]--><!--[--><a class="VPLink link link" href="/articles/rust/15.%E9%AB%98%E7%BA%A7%E7%89%B9%E6%80%A7.html" style="padding-left:0px;" data-v-2cfe069a data-v-3c355974><!--[--><span class="link-text" data-v-2cfe069a>15.高级特性</span><!--]--><!----></a><!----><!--]--><!--[--><a class="VPLink link link" href="/articles/rust/16.%E6%89%8B%E5%86%99web%E6%9C%8D%E5%8A%A1%E5%99%A8%E5%92%8C%E7%BA%BF%E7%A8%8B%E6%B1%A0.html" style="padding-left:0px;" data-v-2cfe069a data-v-3c355974><!--[--><span class="link-text" data-v-2cfe069a>16.手写web服务器和线程池</span><!--]--><!----></a><!----><!--]--><!--[--><a class="VPLink link link" href="/articles/rust/17.rust%E5%BC%82%E6%AD%A5%E7%BC%96%E7%A8%8B.html" style="padding-left:0px;" data-v-2cfe069a data-v-3c355974><!--[--><span class="link-text" data-v-2cfe069a>17.rust异步编程</span><!--]--><!----></a><!----><!--]--><!--]--></div></section></div><div class="group" data-v-f332cb62><section class="VPSidebarGroup collapsible collapsed" data-v-f332cb62 data-v-2976c796><div class="title" role="button" data-v-2976c796><h2 class="title-text" data-v-2976c796>rustweb</h2><div class="action" data-v-2976c796><svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" viewbox="0 0 24 24" class="icon minus" data-v-2976c796><path 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server</span><!--]--><!----></a><!----><!--]--><!--[--><a class="VPLink link link" href="/articles/rustweb/02.Actix.html" style="padding-left:0px;" data-v-2cfe069a data-v-3c355974><!--[--><span class="link-text" data-v-2cfe069a>02.Actix</span><!--]--><!----></a><!----><!--]--><!--[--><a class="VPLink link link" href="/articles/rustweb/03.%E8%BF%9E%E6%8E%A5%E6%95%B0%E6%8D%AE%E5%BA%93.html" style="padding-left:0px;" data-v-2cfe069a data-v-3c355974><!--[--><span class="link-text" data-v-2cfe069a>03.连接数据库</span><!--]--><!----></a><!----><!--]--><!--[--><a class="VPLink link link" href="/articles/rustweb/04.%E9%94%99%E8%AF%AF%E5%A4%84%E7%90%86.html" style="padding-left:0px;" data-v-2cfe069a data-v-3c355974><!--[--><span class="link-text" data-v-2cfe069a>04.错误处理</span><!--]--><!----></a><!----><!--]--><!--[--><a class="VPLink link link" href="/articles/rustweb/05.%E5%AE%8C%E6%95%B4%E9%A1%B9%E7%9B%AE%E9%87%8D%E6%9E%84.html" style="padding-left:0px;" data-v-2cfe069a data-v-3c355974><!--[--><span class="link-text" data-v-2cfe069a>05.完整项目重构</span><!--]--><!----></a><!----><!--]--><!--[--><a class="VPLink link link" href="/articles/rustweb/06.%E6%9C%8D%E5%8A%A1%E5%99%A8%E7%AB%AFweb%E5%BA%94%E7%94%A8.html" style="padding-left:0px;" data-v-2cfe069a data-v-3c355974><!--[--><span class="link-text" data-v-2cfe069a>06.服务器端web应用</span><!--]--><!----></a><!----><!--]--><!--[--><a class="VPLink link link" href="/articles/rustweb/07.WebAssembly.html" style="padding-left:0px;" data-v-2cfe069a data-v-3c355974><!--[--><span class="link-text" data-v-2cfe069a>07.WebAssembly</span><!--]--><!----></a><!----><!--]--><!--[--><a class="VPLink link link" href="/articles/rustweb/08.%E5%8F%91%E5%B8%83.html" style="padding-left:0px;" data-v-2cfe069a data-v-3c355974><!--[--><span class="link-text" data-v-2cfe069a>08.发布</span><!--]--><!----></a><!----><!--]--><!--]--></div></section></div><!--]--></nav></aside><div class="VPContent has-sidebar" id="VPContent" data-v-c6a644e1 data-v-c95df128><div class="VPDoc has-sidebar has-aside" data-v-c95df128 data-v-37ebe389><div class="container" data-v-37ebe389><div class="aside" data-v-37ebe389><div class="aside-curtain" data-v-37ebe389></div><div class="aside-container" data-v-37ebe389><div class="aside-content" data-v-37ebe389><div class="VPDocAside" data-v-37ebe389 data-v-afc4c1a1><!--[--><!--]--><!--[--><!--]--><div class="VPDocAsideOutline" data-v-afc4c1a1 data-v-2865c0b0><div class="content" data-v-2865c0b0><div class="outline-marker" data-v-2865c0b0></div><div class="outline-title" data-v-2865c0b0>On this page</div><nav aria-labelledby="doc-outline-aria-label" data-v-2865c0b0><span class="visually-hidden" id="doc-outline-aria-label" data-v-2865c0b0> Table of Contents for current page </span><ul class="root" data-v-2865c0b0 data-v-1188541a><!--[--><!--]--></ul></nav></div></div><!--[--><!--]--><div class="spacer" data-v-afc4c1a1></div><!--[--><!--]--><!----><!--[--><!--]--><!--[--><!--]--></div></div></div></div><div class="content" data-v-37ebe389><div class="content-container" data-v-37ebe389><!--[--><!--]--><main class="main" data-v-37ebe389><div style="position:relative;" class="vp-doc _articles_rust_03_%E6%8C%87%E9%92%88%E4%B8%8E%E5%86%85%E5%AD%98%E5%88%86%E9%85%8D" data-v-37ebe389><div><h1 id="rust入门（三）：内存与指针" tabindex="-1">Rust入门（三）：内存与指针 <a class="header-anchor" href="#rust入门（三）：内存与指针" aria-hidden="true">#</a></h1><h2 id="rust内存回收" tabindex="-1">Rust内存回收 <a class="header-anchor" href="#rust内存回收" aria-hidden="true">#</a></h2><p>所有程序都必须管理其运行时使用计算机内存的方式。</p><ol><li><p>一些语言中具有垃圾回收机制，比如说java；</p></li><li><p>一些语言需要程序员手动分配和释放内存，比如说c；</p></li><li><p>rust采用了第三种方式，使用所有权管理系统来管理内存</p></li></ol><h2 id="rust内存分配" tabindex="-1">Rust内存分配 <a class="header-anchor" href="#rust内存分配" aria-hidden="true">#</a></h2><p>rust的 栈和堆都是代码在运行时可供使用的内存，它们的结构不同。</p><ul><li>栈中的所有数据都必须占用已知且固定的大小</li><li>堆是缺乏组织的，当向堆放入数据时，你要请求一定大小的空间。内存分配器在堆的某处找到一块足够大的空位，把它标记为已使用，并返回一个表示该位置地址的指针，然后将该指针存储在栈，因为这个指针是固定大小的。</li><li>入栈比在堆上分配内存要快，无需为存储新数据去搜索内存空间；访问堆上的数据比访问栈上的数据慢</li><li>之前提到的所有的基础数据类型（整数、浮点数、元组等等）都存储在栈上，他们的长度是固定的</li><li>后续会介绍一些存储在堆上的可变长度的数据结构，例如Vector、String等，本章节仅用 String 这一个结构来介绍本章节的内容</li></ul><h2 id="string类型" tabindex="-1">String类型 <a class="header-anchor" href="#string类型" aria-hidden="true">#</a></h2><p>String 这个类型管理被分配到堆上的数据，能够存储在编译时未知大小的文本，也可以在初始化后改变存储的数据的长度</p><div class="language-rust"><button class="copy"></button><span class="lang">rust</span><pre><code><span class="line"><span style="color:#676E95;">//使用String::from初始化一个string</span></span>
<span class="line"><span style="color:#C792EA;">let</span><span style="color:#A6ACCD;"> s </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> </span><span style="color:#FFCB6B;">String</span><span style="color:#89DDFF;">::</span><span style="color:#82AAFF;">from</span><span style="color:#89DDFF;">(</span><span style="color:#89DDFF;">&quot;</span><span style="color:#C3E88D;">hello</span><span style="color:#89DDFF;">&quot;</span><span style="color:#89DDFF;">);</span></span>
<span class="line"><span style="color:#676E95;">//push_str这个方法可以追加字符串的内容</span></span>
<span class="line"><span style="color:#A6ACCD;">s</span><span style="color:#89DDFF;">.</span><span style="color:#82AAFF;">push_str</span><span style="color:#89DDFF;">(</span><span style="color:#89DDFF;">&quot;</span><span style="color:#C3E88D;">, world!</span><span style="color:#89DDFF;">&quot;</span><span style="color:#89DDFF;">);</span></span>
<span class="line"></span></code></pre></div><h2 id="rust所有权规则" tabindex="-1">Rust所有权规则 <a class="header-anchor" href="#rust所有权规则" aria-hidden="true">#</a></h2><p>rust的值遵循以下的规则：</p><ol><li>rust 中的每一个值都有一个所有者。</li><li>值在任一时刻有且只有一个所有者。</li><li>当所有者（变量）离开作用域，这个值将被丢弃。</li></ol><p>具体的说明如下：</p><ul><li>作用域机制</li></ul><p>和其他语言类似，rust的作用域也是用 { } 包裹的一个代码块，一个变量从他被声明开始有效，到代码块的尽头结束</p><div class="language-rust"><button class="copy"></button><span class="lang">rust</span><pre><code><span class="line"><span style="color:#89DDFF;">{</span><span style="color:#676E95;">                      // s 在这里无效, 它尚未声明</span></span>
<span class="line"><span style="color:#A6ACCD;">    </span><span style="color:#C792EA;">let</span><span style="color:#A6ACCD;"> s </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">&quot;</span><span style="color:#C3E88D;">hello</span><span style="color:#89DDFF;">&quot;</span><span style="color:#89DDFF;">;</span><span style="color:#676E95;">   // 从此处起，s 是有效的</span></span>
<span class="line"><span style="color:#89DDFF;">}</span><span style="color:#676E95;">                      // 此作用域已结束，s 不再有效</span></span>
<span class="line"></span></code></pre></div><ul><li>数据所有权</li></ul><p>当我们将一个基础数据的变量赋值给另一个变量的时候，实际上是将这个变量的数值给予了另一个变量，这个行为被称为<strong>复制</strong></p><div class="language-rust"><button class="copy"></button><span class="lang">rust</span><pre><code><span class="line"><span style="color:#C792EA;">let</span><span style="color:#A6ACCD;"> x </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">5</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#C792EA;">let</span><span style="color:#A6ACCD;"> y </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> x</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#676E95;">//此时x，y都有效</span></span>
<span class="line"></span></code></pre></div><p>但是我们将String类型做相同的操作的时候，并不能成立，因为在栈上存储的只有String的指针和其他信息，具体数据存储在堆上，这样的操作并没有复制数据，仅仅复制了它的指针的其他信息。</p><div class="language-rust"><button class="copy"></button><span class="lang">rust</span><pre><code><span class="line"><span style="color:#C792EA;">let</span><span style="color:#A6ACCD;"> s1 </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> </span><span style="color:#FFCB6B;">String</span><span style="color:#89DDFF;">::</span><span style="color:#82AAFF;">from</span><span style="color:#89DDFF;">(</span><span style="color:#89DDFF;">&quot;</span><span style="color:#C3E88D;">hello</span><span style="color:#89DDFF;">&quot;</span><span style="color:#89DDFF;">);</span></span>
<span class="line"><span style="color:#C792EA;">let</span><span style="color:#A6ACCD;"> s2 </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> s1</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#676E95;">//这个表述会让s1失效</span></span>
<span class="line"></span></code></pre></div><p>因为此时有两个数据指针指向了同一位置。这就有了一个问题：当 <code>s2</code> 和 <code>s1</code> 离开作用域，他们都会尝试释放相同的内存。这是一个叫做 <strong>二次释放</strong> 的错误，所以rust禁止了这种做法，直接让 <code>s1</code> 失效。这种做法被称为<strong>移动</strong>。</p><p>如果我们想要实现一个堆上类型的复制，我们需要对其数据进行<strong>克隆</strong>，然后创建一个新的指针指向复制出来的数据，类似于 js 中的深拷贝的做法，在rust里使用 clone 这个函数来实现</p><div class="language-rust"><button class="copy"></button><span class="lang">rust</span><pre><code><span class="line"><span style="color:#C792EA;">let</span><span style="color:#A6ACCD;"> s1 </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> </span><span style="color:#FFCB6B;">String</span><span style="color:#89DDFF;">::</span><span style="color:#82AAFF;">from</span><span style="color:#89DDFF;">(</span><span style="color:#89DDFF;">&quot;</span><span style="color:#C3E88D;">hello</span><span style="color:#89DDFF;">&quot;</span><span style="color:#89DDFF;">);</span></span>
<span class="line"><span style="color:#C792EA;">let</span><span style="color:#A6ACCD;"> s2 </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> s1</span><span style="color:#89DDFF;">.</span><span style="color:#82AAFF;">clone</span><span style="color:#89DDFF;">();</span></span>
<span class="line"></span></code></pre></div><ul><li>所有权与函数</li></ul><p>在rust中，将数据传入到函数中和上文提到的句子一样，会产生数据的移动和复制，所以如果我们将一个 String 传入函数，那么我们接下来就不能调用这个 String 了</p><div class="language-rust"><button class="copy"></button><span class="lang">rust</span><pre><code><span class="line"><span style="color:#C792EA;">let</span><span style="color:#A6ACCD;"> s </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> </span><span style="color:#FFCB6B;">String</span><span style="color:#89DDFF;">::</span><span style="color:#82AAFF;">from</span><span style="color:#89DDFF;">(</span><span style="color:#89DDFF;">&quot;</span><span style="color:#C3E88D;">hello</span><span style="color:#89DDFF;">&quot;</span><span style="color:#89DDFF;">);</span><span style="color:#676E95;">  // s 进入作用域</span></span>
<span class="line"><span style="color:#82AAFF;">takes_ownership</span><span style="color:#89DDFF;">(</span><span style="color:#A6ACCD;">s</span><span style="color:#89DDFF;">);</span></span>
<span class="line"><span style="color:#676E95;">//这个操作是不允许的</span></span>
<span class="line"><span style="color:#82AAFF;">println!</span><span style="color:#89DDFF;">(</span><span style="color:#89DDFF;">&quot;{}&quot;</span><span style="color:#89DDFF;">,</span><span style="color:#A6ACCD;">s</span><span style="color:#89DDFF;">);</span></span>
<span class="line"></span>
<span class="line"><span style="color:#C792EA;">let</span><span style="color:#A6ACCD;"> x </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">5</span><span style="color:#89DDFF;">;</span><span style="color:#676E95;">                      // x 进入作用域</span></span>
<span class="line"><span style="color:#82AAFF;">makes_copy</span><span style="color:#89DDFF;">(</span><span style="color:#A6ACCD;">x</span><span style="color:#89DDFF;">);</span></span>
<span class="line"><span style="color:#676E95;">//这个操作就是允许的</span></span>
<span class="line"><span style="color:#82AAFF;">println!</span><span style="color:#89DDFF;">(</span><span style="color:#89DDFF;">&quot;{}&quot;</span><span style="color:#89DDFF;">,</span><span style="color:#A6ACCD;">x</span><span style="color:#89DDFF;">);</span></span>
<span class="line"></span></code></pre></div><p>如果想要继续使用这个变量，可以在函数中返回一个传入的变量，实现一个移动</p><div class="language-rust"><button class="copy"></button><span class="lang">rust</span><pre><code><span class="line"><span style="color:#C792EA;">let</span><span style="color:#A6ACCD;"> s2 </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> </span><span style="color:#FFCB6B;">String</span><span style="color:#89DDFF;">::</span><span style="color:#82AAFF;">from</span><span style="color:#89DDFF;">(</span><span style="color:#89DDFF;">&quot;</span><span style="color:#C3E88D;">hello</span><span style="color:#89DDFF;">&quot;</span><span style="color:#89DDFF;">);</span><span style="color:#A6ACCD;">    </span></span>
<span class="line"><span style="color:#C792EA;">let</span><span style="color:#A6ACCD;"> s3 </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">takes_and_gives_back</span><span style="color:#89DDFF;">(</span><span style="color:#A6ACCD;">s2</span><span style="color:#89DDFF;">);</span><span style="color:#676E95;">  // s2 被移动到s3</span></span>
<span class="line"><span style="color:#F78C6C;">fn</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">takes_and_gives_back</span><span style="color:#89DDFF;">(</span><span style="color:#A6ACCD;">a_string</span><span style="color:#89DDFF;">:</span><span style="color:#A6ACCD;"> </span><span style="color:#FFCB6B;">String</span><span style="color:#89DDFF;">)</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">-&gt;</span><span style="color:#A6ACCD;"> </span><span style="color:#FFCB6B;">String</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span><span style="color:#A6ACCD;"> </span></span>
<span class="line"><span style="color:#A6ACCD;">    a_string</span><span style="color:#676E95;">  // 返回 a_string 并移出给调用的函数</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span>
<span class="line"></span></code></pre></div><h2 id="引用" tabindex="-1">引用 <a class="header-anchor" href="#引用" aria-hidden="true">#</a></h2><p>对于类似于 String 的数据结构，我们可以使用引用来传递这类数值，引用类似 c语言的指针，它是一个地址，它指向某个特定类型的有效值，其使用 &amp; 符号定义。</p><div class="language-rust"><button class="copy"></button><span class="lang">rust</span><pre><code><span class="line"><span style="color:#C792EA;">let</span><span style="color:#A6ACCD;"> s1 </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> </span><span style="color:#FFCB6B;">String</span><span style="color:#89DDFF;">::</span><span style="color:#82AAFF;">from</span><span style="color:#89DDFF;">(</span><span style="color:#89DDFF;">&quot;</span><span style="color:#C3E88D;">hello</span><span style="color:#89DDFF;">&quot;</span><span style="color:#89DDFF;">);</span></span>
<span class="line"><span style="color:#C792EA;">let</span><span style="color:#A6ACCD;"> s2 </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">&amp;</span><span style="color:#A6ACCD;">s1</span><span style="color:#89DDFF;">;</span></span>
<span class="line"></span></code></pre></div><p>在调用函数时，如果传入一个引用，那么获取到的就是引用，而不是数据本身，所以本身的所有权并没有转移，所以不会被销毁。</p><div class="language-rust"><button class="copy"></button><span class="lang">rust</span><pre><code><span class="line"><span style="color:#F78C6C;">fn</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">main</span><span style="color:#89DDFF;">()</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#A6ACCD;">    </span><span style="color:#C792EA;">let</span><span style="color:#A6ACCD;"> s1 </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> </span><span style="color:#FFCB6B;">String</span><span style="color:#89DDFF;">::</span><span style="color:#82AAFF;">from</span><span style="color:#89DDFF;">(</span><span style="color:#89DDFF;">&quot;</span><span style="color:#C3E88D;">hello</span><span style="color:#89DDFF;">&quot;</span><span style="color:#89DDFF;">);</span></span>
<span class="line"><span style="color:#676E95;">	//传入时也需要加上&amp;</span></span>
<span class="line"><span style="color:#A6ACCD;">    </span><span style="color:#C792EA;">let</span><span style="color:#A6ACCD;"> len </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">calculate_length</span><span style="color:#89DDFF;">(&amp;</span><span style="color:#A6ACCD;">s1</span><span style="color:#89DDFF;">);</span></span>
<span class="line"></span>
<span class="line"><span style="color:#A6ACCD;">    </span><span style="color:#82AAFF;">println!</span><span style="color:#89DDFF;">(</span><span style="color:#89DDFF;">&quot;</span><span style="color:#C3E88D;">The length of &#39;</span><span style="color:#89DDFF;">{}</span><span style="color:#C3E88D;">&#39; is </span><span style="color:#89DDFF;">{}</span><span style="color:#C3E88D;">.</span><span style="color:#89DDFF;">&quot;</span><span style="color:#89DDFF;">,</span><span style="color:#A6ACCD;"> s1</span><span style="color:#89DDFF;">,</span><span style="color:#A6ACCD;"> len</span><span style="color:#89DDFF;">);</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span>
<span class="line"><span style="color:#676E95;">//这里接收一个引用类型</span></span>
<span class="line"><span style="color:#F78C6C;">fn</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">calculate_length</span><span style="color:#89DDFF;">(</span><span style="color:#A6ACCD;">s</span><span style="color:#89DDFF;">:</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">&amp;</span><span style="color:#FFCB6B;">String</span><span style="color:#89DDFF;">)</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">-&gt;</span><span style="color:#A6ACCD;"> </span><span style="color:#FFCB6B;">usize</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#A6ACCD;">    s</span><span style="color:#89DDFF;">.</span><span style="color:#82AAFF;">len</span><span style="color:#89DDFF;">()</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span>
<span class="line"></span></code></pre></div><p>引用同样是可以设置为mut的，在设置为 mut 之后，如果我们在函数内部改变引用的值，那么在函数结束后，被传入的值将会改变，因为引用相当于传入了地址，函数对这个地址指向的数据进行了修改，那么再用这个地址寻找数据时，获得的自然是修改后的数据。</p><div class="language-rust"><button class="copy"></button><span class="lang">rust</span><pre><code><span class="line"><span style="color:#F78C6C;">fn</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">main</span><span style="color:#89DDFF;">()</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#A6ACCD;">    </span><span style="color:#C792EA;">let</span><span style="color:#A6ACCD;"> </span><span style="color:#C792EA;">mut</span><span style="color:#A6ACCD;"> s </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> </span><span style="color:#FFCB6B;">String</span><span style="color:#89DDFF;">::</span><span style="color:#82AAFF;">from</span><span style="color:#89DDFF;">(</span><span style="color:#89DDFF;">&quot;</span><span style="color:#C3E88D;">hello</span><span style="color:#89DDFF;">&quot;</span><span style="color:#89DDFF;">);</span></span>
<span class="line"></span>
<span class="line"><span style="color:#A6ACCD;">    </span><span style="color:#82AAFF;">change</span><span style="color:#89DDFF;">(&amp;</span><span style="color:#C792EA;">mut</span><span style="color:#A6ACCD;"> s</span><span style="color:#89DDFF;">);</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span>
<span class="line"></span>
<span class="line"><span style="color:#F78C6C;">fn</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">change</span><span style="color:#89DDFF;">(</span><span style="color:#A6ACCD;">some_string</span><span style="color:#89DDFF;">:</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">&amp;</span><span style="color:#C792EA;">mut</span><span style="color:#A6ACCD;"> </span><span style="color:#FFCB6B;">String</span><span style="color:#89DDFF;">)</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#A6ACCD;">    some_string</span><span style="color:#89DDFF;">.</span><span style="color:#82AAFF;">push_str</span><span style="color:#89DDFF;">(</span><span style="color:#89DDFF;">&quot;</span><span style="color:#C3E88D;">, world</span><span style="color:#89DDFF;">&quot;</span><span style="color:#89DDFF;">);</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span>
<span class="line"></span></code></pre></div><p>但是可变引用是有条件的，如果有多个可变引用的话，可能会产生同时有多个逻辑修改同一片地址的情况；如果同时存在可变引用和不可变引用的话，不可变引用调取的内容可能被可变引用改变。所以引用的条件是</p><ul><li>同时只能有一个可变引用，在拥有了一个可变引用之后，不能在拥有其他引用</li><li>可以同时拥有多个不可变引用</li></ul><div class="language-rust"><button class="copy"></button><span class="lang">rust</span><pre><code><span class="line"><span style="color:#676E95;">//这是不允许的</span></span>
<span class="line"><span style="color:#C792EA;">let</span><span style="color:#A6ACCD;"> </span><span style="color:#C792EA;">mut</span><span style="color:#A6ACCD;"> s </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> </span><span style="color:#FFCB6B;">String</span><span style="color:#89DDFF;">::</span><span style="color:#82AAFF;">from</span><span style="color:#89DDFF;">(</span><span style="color:#89DDFF;">&quot;</span><span style="color:#C3E88D;">hello</span><span style="color:#89DDFF;">&quot;</span><span style="color:#89DDFF;">);</span></span>
<span class="line"><span style="color:#C792EA;">let</span><span style="color:#A6ACCD;"> r1 </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">&amp;</span><span style="color:#C792EA;">mut</span><span style="color:#A6ACCD;"> s</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#C792EA;">let</span><span style="color:#A6ACCD;"> r2 </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">&amp;</span><span style="color:#C792EA;">mut</span><span style="color:#A6ACCD;"> s</span><span style="color:#89DDFF;">;</span></span>
<span class="line"></span></code></pre></div><p>同时，在其他语言中可能出现以下情况，一个引用指向的内容被释放了，导致某给引用指向了空值的情况，在rust中，不允许这种情况的存在，rust必须保证你引用指向的对象一直有效。</p><div class="language-rust"><button class="copy"></button><span class="lang">rust</span><pre><code><span class="line"><span style="color:#676E95;">//这个例子是不能通过编译的</span></span>
<span class="line"><span style="color:#F78C6C;">fn</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">main</span><span style="color:#89DDFF;">()</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#A6ACCD;">    </span><span style="color:#C792EA;">let</span><span style="color:#A6ACCD;"> reference_to_nothing </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">dangle</span><span style="color:#89DDFF;">();</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span>
<span class="line"><span style="color:#F78C6C;">fn</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">dangle</span><span style="color:#89DDFF;">()</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">-&gt;</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">&amp;</span><span style="color:#FFCB6B;">String</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#A6ACCD;">    </span><span style="color:#C792EA;">let</span><span style="color:#A6ACCD;"> s </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> </span><span style="color:#FFCB6B;">String</span><span style="color:#89DDFF;">::</span><span style="color:#82AAFF;">from</span><span style="color:#89DDFF;">(</span><span style="color:#89DDFF;">&quot;</span><span style="color:#C3E88D;">hello</span><span style="color:#89DDFF;">&quot;</span><span style="color:#89DDFF;">);</span></span>
<span class="line"><span style="color:#A6ACCD;">    </span><span style="color:#89DDFF;">&amp;</span><span style="color:#A6ACCD;">s</span></span>
<span class="line"><span style="color:#89DDFF;">}</span><span style="color:#676E95;">//这里s的作用域结束了</span></span>
<span class="line"></span></code></pre></div><h2 id="slice类型" tabindex="-1">Slice类型 <a class="header-anchor" href="#slice类型" aria-hidden="true">#</a></h2><p>slice允许你引用集合中一段连续的元素序列，而不用引用整个集合，slice 是一类引用，所以它没有所有权。</p><p>字符串slice可以截取字符串中的一部分，可以使用一个由中括号中的 <code>[starting_index..ending_index]</code> 指定的 range 创建一个 slice，包含开始位置不包含结束位置</p><div class="language-rust"><button class="copy"></button><span class="lang">rust</span><pre><code><span class="line"><span style="color:#C792EA;">let</span><span style="color:#A6ACCD;"> s </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> </span><span style="color:#FFCB6B;">String</span><span style="color:#89DDFF;">::</span><span style="color:#82AAFF;">from</span><span style="color:#89DDFF;">(</span><span style="color:#89DDFF;">&quot;</span><span style="color:#C3E88D;">hello</span><span style="color:#89DDFF;">&quot;</span><span style="color:#89DDFF;">);</span></span>
<span class="line"><span style="color:#C792EA;">let</span><span style="color:#A6ACCD;"> slice </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">&amp;</span><span style="color:#A6ACCD;">s</span><span style="color:#89DDFF;">[</span><span style="color:#F78C6C;">0</span><span style="color:#89DDFF;">..</span><span style="color:#F78C6C;">2</span><span style="color:#89DDFF;">];</span></span>
<span class="line"></span></code></pre></div><p>如果直接定义字符串的字面值就是slice类型。如果想要将其传递到函数中，需要用 &amp;str 来作为它的类型</p><div class="language-rust"><button class="copy"></button><span class="lang">rust</span><pre><code><span class="line"><span style="color:#C792EA;">let</span><span style="color:#A6ACCD;"> s </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">&quot;</span><span style="color:#C3E88D;">Hello, world!</span><span style="color:#89DDFF;">&quot;</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#F78C6C;">fn</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">first_word</span><span style="color:#89DDFF;">(</span><span style="color:#A6ACCD;">s</span><span style="color:#89DDFF;">:</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">&amp;</span><span style="color:#FFCB6B;">str</span><span style="color:#89DDFF;">)</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">-&gt;</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">&amp;</span><span style="color:#FFCB6B;">str</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{}</span></span>
<span class="line"></span></code></pre></div></div></div></main><!--[--><!--]--><footer class="VPDocFooter" data-v-37ebe389 data-v-a54a85bd><!----><div class="prev-next" data-v-a54a85bd><div class="pager" data-v-a54a85bd><a class="pager-link prev" href="/articles/rust/02.%E5%9F%BA%E7%A1%80%E8%AF%AD%E6%B3%95.html" data-v-a54a85bd><span class="desc" data-v-a54a85bd>Previous page</span><span class="title" data-v-a54a85bd>02.基础语法</span></a></div><div class="has-prev pager" data-v-a54a85bd><a class="pager-link next" href="/articles/rust/04.%E7%A8%8B%E5%BA%8F%E7%BB%93%E6%9E%84%E5%92%8C%E5%BC%82%E5%B8%B8%E5%A4%84%E7%90%86.html" data-v-a54a85bd><span class="desc" data-v-a54a85bd>Next page</span><span class="title" data-v-a54a85bd>04.程序结构和异常处理</span></a></div></div></footer><!--[--><!--]--></div></div></div></div></div><!----><!--[--><!--]--></div></div>
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